EN / DA
Body and mind

Weight-regulating hormone weakens the bones of obese people

People’s bones are automatically strengthened when they are subjected to weight-bearing loads. Researchers have therefore been puzzled as to why obese people are still twice as likely to experience bone fractures as people with normal weight. A new study of young obese people reveals that a hormone apparently weakens the bones of obese people. The study also suggests that different mechanisms apply to men and women.

Obese people naturally experience more bone fractures. Although obese people are twice as likely to have bone fractures as people with normal weight, it is far from given. On the contrary, this has been somewhat of a paradox within medicine since weight bearing is known to strengthen bone density and mass. Now a Finnish and Swedish collaboration has explained why obese people’s bones more often fracture despite their weight.

“Obese people are affected by a whole range of bodily systems that are out of balance. Ascertaining what causes their bones to weaken has therefore been extremely complex. Our new study clearly shows that excessive quantities of the weight-regulating hormone leptin associates inversely with bone turnover and impaired bone quality in men. The mechanism for women seems to differ slightly and is initiated by C-reactive protein,” explains Heli T. Viljakainen, Children’s Hospital, University of Helsinki.

When bones become porous

The researchers extensively studied 55 young people with a body mass index exceeding 40 and an average age of 20 years. They compared the concentrations of important metabolic substances in the blood with those among 65 people with normal weight. These concentrations were compared both with bone densitometric measures and circulating bone turnover markers .

“Human bones are continually being remodelled, with old bone tissue breaking down and being resorbed and new tissue forming. Our study showed that the obese participants had severely suppressed bone remodelling. This might result in accumulation of microscopic cracks in their bones that weakened the overall stability.”

Among adults, 10% of healthy bone tissue is remodelled in one year. Every day, bone is stimulated to renew itself through weight-bearing exercise. The greater the load, the stronger the bone becomes. So one might expect the bones of obese people to be remodelled even more than the bones of people with normal weight.

“Unfortunately, this positive effect appears to be offset and then turns negative because leptin apparently influences both the formation of new bone tissue and also the resorption and reuse of the old bone tissue. So this meant that bones are not repaired/maintained and thus fracture more easily.”

Satiety and inflammation

Leptin is formed in fatty tissue as a response to rising amounts of fatty acids in fat cells. Leptin affects the brain and normally produces a sense of satiety and increases the metabolism of energy. However, mutations in the leptin gene among obese people can result in it is not functioning properly, which is why they continue eating despite these hormonal signals and become or remain inactive.

“Among obese men, leptin apparently also has the unfortunate effect of weakening bones. The mechanism differed slightly among obese women, with C-reactive protein initiating this. The chronic inflammation obese people often experience stimulates the production of this protein.”

Although the mechanism seems to differ slightly for obese women, it is closely associated, because C-reactive protein can bind to leptin and thereby block the effect of both substances. The researchers do not yet fully understand the mechanism behind bone loss. Nevertheless, this additional knowledge offers new prospects for diagnosing and treating obese people.

“Our results provide opportunities to assess the fracture risk among obese people. We naturally hope we can eventually find ways to strengthen their bone density and structure. However, the results reinforce our belief that the ideal way to avoid both obesity and osteoporosis is through exercise and movement.”

Metabolic milieu associates with impaired skeletal characteristics in obesity” has been published in PLoS One. The Novo Nordisk Foundation provided a grant to co-author Outi Mäkitie for research in 2013–2016 on the Genetic Causes of Early-onset Osteoporosis.

Heli Viljakainen
Folkhälsan Research Centre , Helsinki · Genetic Research Program Epidemiology, Nutrition and Dietetics, Public Health Docent, PhD (nutrition)